(base) Give a hint in web interface if SimpleXML (php-xml) is missing

[joshtrichards]

• Resolves: #2180
• And prevents similar reports:
  • #31473
  • #23970
  • #18610
  • #15708

Summary

Avoids a 500 error and also gives a useful error message on the web interface if this module isn't installed, gets overlooked during a PHP upgrade, etc.

• While we check for it later, it's too late for session.
• Inspired by #17163
• Related to #2619 (which was never merged and seemed intended to silence things rather than give the user a useful hint for self-resolution)

TODO

• [ ] Maybe some field testing?

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)

[joshtrichards]

Test failures appear unrelated. jsunit had a bunch of setup errors.

[joshtrichards]

The CLI will behave how it does now - i.e. it'll generate an Error: Call to undefined function OC\App\simplexml_load_file() in... stack trace.

I suppose this is an opportunity to provide a friendly message at the command-line too, but the stack trace is at least visible there...